Transducer having adjustable electrical impedance



L. W. CAMP Aug. 21, 1956 TRANSDUCER HAVING ADJUSTABLE ELECTRICALIMPEDANCE Filed Oct. 8, 1951 ATTORNEY I United States Patent TRANSDUCERHAVING ADJUSTABLE ELECTRICAL EDAN CE Leon W. Camp, Glendale, Califl,assignor to Bendix Aviation Corporation, North Hollywood, Cahfi, acorporation of Delaware Application October 8, 1951, Serial No. 250,300

4 Claims. Cl. 340- This invention relates to transducers utilizing anelectromechanically sensitive element having opposed electrodes forapplying or taking off electrical potential.

An object of the invention is to vary the effective electrical impedanceof transducers of this type wlthout altering the dimensions andvibrational characteristics of the electromechanical element.

Another object is to enable the connection 1n seriesparallel relation ofa prime number of similarly d1mensioned transducer elements ofelectro-mechanically sens1 tive type.

Other more specific objects and features of the invention will appearfrom the description to follow:

Electro-mechanically sensitive transducer elements comprise a body ofelectro-mechanically sensitive material having opposed electrodes onopposite sides thereof. The

electrical impedance between the electrodes is a function of thedimensions of the body, which also determine its frequency of mechanicalvibration. As heretofore used, it has not been possible to alter theelectrical impedance of an element without altering the shape ordimensions of the electro-mechanically sensitive body.

In accordance with the present invention I increase the electricalimpedance of an electro-mechanically sensitive transducer element bydividing each electrode into a plurality of sections electricallyinsulated from each other, and connect each pair of opposed sections ofthe electrodes in series with the other pair or pairs of opposedsections, thereby multiplying the impedance in accordance with thesquare of the number of sections into which the electrodes aresubdivided.

Further in accordance with the invention, the subdivision of theelectrodes and their connection in series is applied to one element of amulti-unit transducer to enable the connection of the various elementsin seriesparallel relation where it would be impracticable to do sootherwise. Thus it is often desirable to employ, in a single transducer,a planar array consisting of a central element surrounded by sixelements all of the same size. Since seven is a prime number, it is notpossible to break seven transducerelements down into identicalsub-groups for series-parallel connection. Hence the seven elements hadto be connected all in parallel or all in series. The impedance with theseries connection is seven times that of a single. element and withparallel connection is one-seventh that of a single element. Hence theimpedance of a transducer in which the elements are connected in seriesis forty-nine timesthat with the parallel connection. Often anintermediate impedance would be more desirable.

By splitting the electrodes of the central element into two pairs, andconnecting them in series, the impedance of the central unit can bequadrupled. The six remaining elements can be connected in one or theother of two possible series-parallel patterns to have an impedance ofeither two-thirds or three-halves that of a single unit. With the firstseries-parallel pattern, equal disribution of energy in all sevenelements can be obtained by connecting the two electrode sections of thecentral element in series with each other and in parallel with theseriesparallel pattern of the other six elements; and the overallimpedance will be four-sevenths that of a single element four timesgreater than with all the elements connected in parallel with eachother.

With the six outer elements connected in the alternative series-parallelpattern, having an impedance three-halves that of a single element, theoverall impedance is twelveelevenths that of a single element, and theenergy distribution is greater in the central element than in each ofthe six surrounding elements. Thi is desirable in some installations, asit reduces the magnitude of the side lobes of the directivity pattern ofthe transducer.

A complete understanding of the invention can be had from the followingdetailed description with reference to the drawing, in which:

Fig. 1 is a longitudinal section through a single element transducer inaccordance with the invention;

Fig. 2 is a front elevational view of the vibrator element of thetransducer of Fig. 1;

Fig. 3 is a front elevational view of a multi-element transducer similarto the transducer of Figs. 1 and 2 but having seven elements in a planararray instead of a single element;

Fig. 4 is a schematic diagram showing one manner of interconnecting thevarious elements of the transducer shown in Fig. 3; and

Fig. 5 is a schematic diagram showing an alternative method ofinterconnecting the elements of the transducer shown in Fig. 3.

Referring first to Fig. l the transducer therein disclosed comprises acup-shaped casing 10 having an open end which is closed by a soundtransparent window 11 of rubber or the like. The window is shownretained in position on the casing by a band clamp 12. The casing 10contains a single electro-mechanically sensitive element which may be inthe form of a ring 13 of a ceramic such as barium titanate. The ring 13may be backed by a solid disc 14 of steel or the like, and both the ring13 and disc 14 may be supported within the casing 10 by a mass of soundabsorbent material 15 such as rubber, a mixture of cork and rubber, orthe like. The space within and in front of the ring 13 is filled with asuitable liquid 16 which serves to conduct vibrations from the innerannular surface of the ring 13 to and through the window 11 to the wateror other liquid in which the transducer may be immersed.

Referring to Figs. 1 and 2, the inner annular face 13a of the ring 13 iscovered with an inner electrode consisting of two semi-cylindricalsections 17a and 17b, and the outer annular surface 13b is covered withtwo semi-cylindrical electrode sections 18a and 18b. This constructiondifIers from the prior art construction in that heretofore the innerelectrode has consisted of a single element, and the outer electrode hasconsisted of a single element. The electrode sections 17a and 17bsubstantially completely cover the inner surface 13a of the ring,sufiicient spacing being left between the two sections only to provideelectrical insulation therebetween. The same is true of the two outerelectrode sections 18a and 18b.

The two electrode sections 17b and 18b are connected in series-aidingrelation with the two sections 18a and 17a. Thus one of the transducerleads 20 is connected directly to the outer electrode section 18b, andthe corresponding inner electrode section 17b is connected by a jumperlead 21 with the other outer electrode 18a. The inner electrode 17a isconnected by a jumper lead 22 to the disc 14 which in turn is connectedto the other electrode lead 23.

In the transducer of Figs. .1 and 2, the electrical impedance betweenthe transducer leads'20 and 23 is four times as great as it-would be ina conventional trans ducer of the same type employing single inner andouter electrodes. This will be readily apparent, since the twojuxtaposed electrode sections 17b and 18b have only half the area of afull electrode section, the impedan'ce between these electrodes and alsothe impedance between the electrode sections 17a and 18a is twice thatbetweenthe inner and outer electrodes of a conventional construction.Since the two pairs of sections, eachhaving an impedance twice that ofthe usual arrangement, are connected in series with each other, thetotal impedance is againdoubled.

The invention as applied to a single element transducer as illustratedis often useful because the impedance of the electrical circuit to whichthe transducer is connected may be higher than that of the conventionaltransducer, and it is desirable in order to obtain highest efl'lciencythat the impedance of the transducer match the impedance of theelectrical circuit to which it is connected.

However the invention has special utility in multielement transducerssuch as that disclosed in Fig. 3. This transducer contains "seven rings.25, each corresponding to the ring 13 of Fig. 1. One of therings 25a ispositioned in the center and the other six rings 25b are distributedaround the central ring in as close ,proximity thereto and to each otheras .is possible without actual contact. It is well vknown that an arrayof seven identical rings provides a particularly close-positioning oftherings that cannot be obtained with an other number than seven andthat has proved highly desirable in transducers where directivity isdesired. Heretofore it has been the practice to use a single inner and asingle outer electrode on each of all seven rings of an array such asshown in Fig. 3, and it was possible to interconnect these electrodesonly in series or in parallel. As pointed out hereinabove, theoverallimpedance when all the rings .are connected in parallel is one-sevenththat of a single ring, and when all 'of the rings are connected inseries the impedance is seventimes that of a s'inglering, and forty-ninetimes that of attransducer in which the rings are connected in parallel.It .is not possible to use seven identical rings and break them downinto a series-parallel connection vbecause of the fact that seven is aprime number, i. c. it is not divisibleby anything other than seven andone.

In accordance with the present invention ,a single inner and a singleouter electrode is used on each :of :the six outer rings 25b butmulti-section electrodes suchras shown in Fig. 2. are used on thecentral element 25a.

Because of thetfact that the six outer elements 25b can be broken downinto .eitherthree groups of 'twos or two groups of threes, thecircuitsshown ,in :Figs. 4 a-nl arepossible. In each circuit the two sets ,ofelectrodes on the central element 25a areconnected in series with eachother between the line terminals :27 and. .28.

In Fig. 4 the six elements 25b are arranged in two groups of three each.The three elements in;-each group are connected in parallel with respectto each other, and the two I groups are connected in series with-reachother between the line terminals 27 and 28.

In .Fig. 5 the sixaelements 25b areiarranged inithree groups of two eachand the three groups are connected in series with each other betweenrthe ,line terminals -27.and.28.

"Referring first to Fig. 4, .let ZA be the impedance of the branch Acontaining .the :element 25a having sectioned electrodes connected inseries, and let Z13 be the impedance of the branch B containing the sixelements 25b.

Z ,1 w 4Z 2Z Z B-- E I A Where:

Z=impedance of a single element 25b. E=potential between terminals 27and 28. IA=current in branch A. IB==current in branch B.

As is well known, power W at any given instant is computable from theequation:

tW=El Hencethe power WA in branch A is:

t E E WA E 12-47 and the power W]; in branch B is:

I a all? WE 22 22 Since onlytone-ssixth of the power in branch .Bis.dissipated in each element 25b, the power in each element 25b is:

was a? Since all the .power in :branch A is dissipated in element 25a,the'ratio .of :th'epower in element 25a to .eachlclement 25b .is:

2 E2 in 1 Z2 In --other words, all seven elements areequally-energ'ized.

The impedance Z4 of the complete transducer with the connections shownin Fig. -4 is obtained from the equation:

1 1 1 Z ZA ZB Substituting forthe values of ZA and Zn:

1 1 3 27457252 Solving:

t-lntl-lig. 5,, the impedance :of, .and :the :power :in branch A :is the:same .as i in .tFig. 4, but the'impedance aof branch B is greater, andthe ;power1is Jess.

Thus the impedance 10f branch :3 ;is

and the overall impedance of thetcomp'lete transducer is 22. 1'1

Hie-current in :"branch B is The power in each element 25b is one-sixthof this, or

and the ratio' of the power in the central element 25a to that in eachouter element is As previously stated, it is often desirable to work theouter six elements as a lower level than the center element to reducethe magnitude of side lobes in the directivity pattern.

Although for the purpose of explaining the invention, particularembodiments thereof have been shown and described, obvious modificationswill occur to a person skilled in the art, and I do not desire to belimited to the exact details shown and described.

I claim:

1. In a transducer having a pair of electrical terminals: anelectromechanically responsive vibratable dielectric element foracoustic coupling with a fluid medium, said element having a pair ofjuxtaposed faces; an electrode on each of said faces of said element,each electrode consisting of a plurality of sections electricallyisolated from each other and each section being juxtaposed to and pairedwith a corresponding section of the other electrode; and meansconnecting each pair of sections in series aiding relation with theother pair of sections in and the power is a network to said terminals,whereby the potential gradient in all portions of said dielectricelement is of the same polarity at any instant.

2. A transducer according to claim 1 having in addition to saidmentioned dielectric element a plurality of otherelectromechanically-responsive vibratable dielectrio elementssymmetrically disposed about said firstmentioned element and formingtherewith a directive array; a single section electrode on each face ofeach of said other elements; and means independent of said firstmentioned network connecting the electrodes of said other elements in asecond network between said terminals.

3. A transducer according to claim 2 in which said other elements aresix in number and said second network comprises two groups of elementsconnected in series between said electrical terminals, each of saidgroups comprising three elements connected in parallel with respect toeach other.

4. A transducer according to claim 2 in which said other elements aresix in number and said second network comprises three groups of elementsconnected in series between said electrical terminals, each of saidgroups comprising two elements connected in parallel with respect toeach other.

References Cited in the file of this patent UNITED STATES PATENTS1,869,556 Giebe Aug. 2, 1932 2,269,403 Williams Jan. 6, 1942 2,420,864Chilowsky May 20, 1947 2,427,062 Massa Sept. 9, 1947 2,515,446 GravelyJuly 18, 1950 2,549,872 Willard Apr. 24, 1951

